The desirability of utilizing low rank coals such as the bituminous, sub-bituminous and lignite coals as a replacement for oil has long been recognized. However, many deposits of these coals have a high content of volatile constituents and water which detract from their economic value as a fuel. This is especially important in certain geographical areas, where the mined coal must be shipped over long distances. Additionally, such coals have a high content of impurities, such as sulfur, which make their use environmentally undesirable.
In the prior art, of which I am aware, various processes and apparatuses have been disclosed which have attempted to provide for the drying and purifying of low rank coals.
The most prominent process is the "Fleissner Process" disclosed in U.S. Pat. No. 1,632,829 and No. 1,679,078 both issued to H. Fleissner. The Fleissner process is a batch process which involves the use of saturated steam processing, under high pressure, to remove water from low rank coals. The Fleissner process has operated commercially in Europe to upgrade lignite since 1927.
Several attempts have been made to adapt the "Fleissner Process" for continuous processing. The U.S. Bureau of Mines has developed such an adaptation wherein the continuous processing of lignites is performed at 1500 psig. See Oppelt, W. H., W. R. Kube and T. W. Kamps. "Drying North Dakota Lignite to 1500 Pounds Pressure by the Fleissner Process". BuMines RI 5527, 1959.
U.S. Pat. No. 4,052,168, No. 4,127,391 and No. 4,129,420 all issued to Koppelman also teach adaptations of the "Fleissner Process" for upgrading lignites, bituminous fines and cellulosic materials, respectively. In each of these processes, the desired matter is dried by autoclave treatment for, preferably, 15 minutes to one hour at very high pressures (1,000-3,000 psi) and very high temperatures (750.degree. F. minimum with 1,000.degree.-1,250.degree. F. being preferable). Each of these processes are directed particularly to batch-type autoclaves.
U.S. Pat. No. 4,126,519 issued to Murray discloses an apparatus and method for thermal treatment of organic carbonaceous material. Utilizing a highly specialized apparatus, carbonaceous material in the form of a slurry is preheated and then dried at elevated temperatures (950.degree. F.) and pressures (1,495 psig). The efficiency and capacity of this '519 patent is severely limited by the moisture content present in the material sought to be dried. Moreover, the waste water extracted from the equipment contains environmentally undesirable dissolved organic constituents, which necessitates treatment of the waste water.
U.S. Pat. No. 4,477,257 also issued to Koppelman discloses an apparatus and process for thermal treatment of organic carbonaceous materials. Utilizing a highly specialized apparatus in this complicated process, before drying, the material is first subjected to a preheating stage for 3-60 minutes requiring temperatures of 300.degree.-500.degree. F. and a pressurized dewatering stage. The material is then dried in a reaction stage for 1-60 minutes at high temperatures (400.degree. F.-1,200.degree. F.) and high pressures (300-3,000 psi).
U.S. Pat. No. 3,977,947 issued to Pyle discloses a continuous process for the drying and carbonizing of particulate woody materials. Particulate woody materials are injected on a continuous basis into a gas fluidized bed of previously carbonized materials. The particulate woody material is dried and carbonized to form a solid pyrochar on the surface of the bed. Off-gases with entrained charcoal fines are removed from above the bed and separated in a cyclone system whereby a gaseous fuel is obtained.
U.S. Pat. No. 3,520,795 issued to Schulman, et al teaches a process for retorting oil shale employing externally generated superheated steam in a once through mode. In particular, that process is directed to control of temperatures while eliminating the use of a substantial amount of recycle gas streams in oil shale retorting. That process is also concerned with the use of liquid cooling streams in retorting of oil shale.
U.S. Pat. No. 4,291,539 issued to Potter discloses a power plant wherein steam generated from burning coal in a boiler drives a high-pressure turbine. De-superheated steam from the turbine is then channeled to a dryer where, in the absence of all other gases, it is used to dry moist coal. The dried coal is then utilized to further fuel the boiler. In this process the "dirty steam" generated from drying the coal is vented to the atmosphere. The drying is essentially a "once through mode".
Other prior art patents known to the applicant are as follows:
______________________________________ No. Inventor(s) Year of Issue ______________________________________ 2,579,397 Roetheli 1951 3,001,916 Cheadle 1958 3,061,524 Savage 1962 3,112,255 Champion 1963 3,133,010 Irish, et al. 1964 3,441,394 St. Clair 1969 3,463,623 Forney, et al. 1969 4,104,129 Fields, et al. 1978 4,158,697 Cramer 1979 4,162,959 Duraiswamy 1979 4,274,941 Janssen, et al. 1981 4,278,445 Stickler, et al. 1981 4,331,529 Lambert, et al. 1982 4,359,451 Tipton 1982 4,366,044 Swanson 1982 4,383,912 Saadi, et al. 1983 ______________________________________
Additionally, the processes utilized for treating lignite have been summarized in a publication by the U.S. Department of Energy, Technical Information Center. See Stanmore, B., D. N. Baria and L. E. Paulson, "Steam Drying Of Lignite": A review of Processes and Performance", 1982.
While the processes and apparatus disclosed in the prior art for drying and purifying low rank coals and peat are widespread, these process and apparatuses have several disadvantages and deficiencies, which have severely limited their use, and which may be enumerated as follows:
First, the processes disclosed are carried out using extremely high pressures. Such high pressure requirements demand an energy input which generally make those processes economically undesirable. For example, the Bureau of Mines process is performed at 1,500 psig, while the Koppelman processes require pressures of 1,000-3,000 psi with the higher pressures being preferable. These high pressure requirements also severely reduce the flexibility of those processes and increase the inherent risks and dangers associated therewith.
Second, the processes of the prior art are all carried out using extremely high temperatures. For example, the Koppelman processes disclose preferable temperatures of 1,000.degree. F.-1,200.degree. F. Such high temperature requirements demand an energy input which aids in rendering those processes economically undesirable.
Third, the processes of the prior art require that the matter to be dried be subjected to the aforementioned high temperatures and pressures for prolonged periods of time (referred to as residence times). For example, the Koppelman processes disclose usual residence times of from 15 minutes to one hour. These extended residence times not only increase the amount of energy input into the system, but also reduce the amount of product which can be processed over a given period of time, thereby further rendering those processes economically undesirable.
Fourth, the processes disclosed require specialized and expensive equipment, apparatuses, and facilities which increase capital investment and production costs, thereby further rendering those processes economically undesirable.
Fifth, the processes of the prior art generally do not provide capabilities to sufficiently remove impurities such as ash, sulfur and pyrite from the coal. Therefore, to comply with federal, state and local environmental regulations, it has become customary in the prior art to mix the fuels produced by those processes with imported low sulfur fuels to provide a residual blend having a lower sulfur content. The high cost of importing such fuels further renders these processes economically undesirable.
Sixth, the processes of the prior art (such as the aforementioned Murray patent) produce waste water which contain environmentally undesirable dissolved organic constituents. To comply with environmental regulations, such waste water must be treated requiring additional equipment, facilities and time, thereby increasing the costs involved with those processes and rendering them economically unfeasible.
Finally, in the processes of the prior art, hydrocarbons are evaporated along with the drying gases in concentrations which are too low to economically permit recovery. Accordingly, these gases are evaporated to the atmosphere. Consequently, these processes are environmentally undesirable.